1. Field of the Invention
The invention generally relates to a method of inhibiting polymerization of vinyl-aryl monomers, a process for increasing number average molecular weight of a polystyrene, and to an inhibitor composition useful therein.
2. Background Art
Vinyl-aryl monomers can be polymerized to prepare aryl-containing polymers, which contain repeat units comprising residuals of the vinyl-aryl monomers. Examples of the vinyl-aryl monomers are styrene and divinylbenzenes. Examples of the aryl-containing polymers that can be prepared therefrom are polystyrene, poly(acrylonitrile-butadiene-styrene), poly(styrene-butadiene) rubber, poly(styrene-butadiene) latex, poly(styrene-isoprene-styrene), poly(styrene-divinylbenzene), polydivinylbenzene, and poly(styrene-divinylbenzene)-containing polyesters.
These and other aryl-containing polymers have found a wide variety of commercial uses. Examples of such commercial uses are building insulation, synthetic rubber, thermoplastics, fiberglass, piping, automobile and boat parts, food containers, carpet backing, and ion exchange resins.
Oftentimes, however, the vinyl-aryl monomers prematurely polymerize before they can be properly used to prepare the aryl-containing polymers. This premature or unwanted polymerization can occur throughout the lifetimes of the vinyl-aryl monomers starting from a time just after their synthesis and continuing through their subsequent purification, storage, and transportation. The unwanted polymerization prematurely consumes the vinyl-aryl monomers and leads to polymer and gel formation and fouling of equipment such as distillation columns.
The fouling of equipment has been a major problem in the chemicals and polymer industries. This is because manufacturing downtime caused by the fouling of equipment increases waste and manufacturing costs and can decrease product quality.
The chemicals and polymer industries have tried using inhibitors of polymerization of vinyl-aryl monomers to prevent such fouling, but with limited success. One commonly used inhibitor appears to be 2-(secondary-butyl)-4,6-dinitrophenol (DNBP), also known as 4,6-dinitro-2-(1-methylpropyl)-phenol and Dinoseb (CAS no. 88-85-7). The DNBP is used in liquid phases, but is toxic to humans and animals.
The premature polymerization of styrene in particular has also meant that it has been difficult to prepare high molecular weight polystyrene by a radical polymerization process. Ideally polymerization of styrene would be initiated by generating few styrene radicals, and then the few styrene radicals would propagate by consuming remaining styrene monomer to form a small number polystyrene chains until a high molecular weight polystyrene is formed. Unfortunately heating styrene monomer causes too many styrene radicals to spontaneously form. It is believed that these many styrene-derived radicals lead to a substantial increase in numbers of polystyrene chains, thereby undesirably leading to low molecular weight polystyrene.
There is a need in the chemicals and polymer industries for an improved method of inhibiting unwanted polymerization of the vinyl-aryl monomers until such time that polymerization thereof is desired, and for improved inhibitor compositions useful therein. There is also a need in the polymer industry for a process for preparing high molecular weight polystyrene via a radical polymerization process.